This invention relates to bidirectional triode thyristors and, more particularly, to improving commutation characteristics of triode thyristors.
Since its introduction, the bidirectional triode thyristor, or triac, has been growing in popularity among circuit designers. The reasons for this growth are evident; as a single component that can replace several components in many circuit applications, the triac can provide improvements in weight, size and cost. However, improvement is still being sought in some characteristics of triac performance. Consider, for example, the commonly used circuit in which a triac replaces two inversely connected parallel SCR's to selectively pass AC current. When using the SCR's each device has an entire half cycle in a back biased state to turn off. Thus, false firing due to the presence of mobile carriers when forward voltage is reapplied is rarely a problem. However, consider the corresponding triac circuit. One region of the triac pellet is conductive during one half cycle and another region is conductive during the following half cycle. See, for example, the SCR Manual, 5th Edition, copyright 1972 by the General Electric Co., or U.S. Pat. No. 3,275,909. Consequently, in the AC application, some portion of the triac is in a conductive state at all times except during brief periods near the crossovers of the AC input signal. If mobile charge carriers from the conductive portion "spill over" into the non-conductive portion, the non-conductive portion can be inadvertently triggered by those carriers on the reversal of the applied voltage. This effect has limited the frequencies at which triacs can be used.
It is, therefore, an object of this invention to provide a triac which resists carrier spillover from the conductive region to the non-conductive region.